As I understand it, degradation of this area by humans eliminated all the
grapevines that gave these hills their name. This is, unfortunately, a common
situation in the Big Bend area, which apparently used to be much more amenable
to human habitation. The problem seems to have been mostly overgrazing by early
ranchers. Fortunately, the rocks remain, and in these hills erosion has
created a truly grotesque landscape - a terrain that should be on a planet
other than earth. The Native American legend that Big Bend was the dumping
ground for what was left over after the creation of the world can almost be
believed when you take this hike.

Before you hike (virtually speaking) up the draw the trail follows, check
out the view northwestward toward the Rosillos Mountains (below).

In the distance you see the south side of Tornillo Creek. The layered
structure
in the side of the creek shows its banks consist of previously deposited
sediments derived from the Rosillos Mountains to the north. This sediment
(alluvium) was
deposited over the past several thousand years, but now, possibly due to either
a decrease in rainfall or an increase in overall elevation (or both), it is
being worn away. The dominant regime of the Big
Bend area is one of erosion, so the deposition indicated in the arroyo's banks
is but a glitch in the overall trend to wear down the landscape. Also seen in
this picture is a small
igneousintrusion, probably just
the tip of a larger body of intrusive igneous rock that resides mostly
subsurface.

As you stroll up the easy trail you are increasingly surrounded by these
odd roundish oblong rocks that extend all the way up the
ridges on either side. It is almost like being in a stadium or coliseum with
all these rock spectators watching you make your way up to the Head Rock (who
lives at the end of the trail - more about him later). Below is a view seen
early on in the hike.

What is responsible for these strange rock shapes? First of all, the
Grapevine Hills is an igneous
laccolith (a
mushroom-shaped body of igneous rock) exposed by erosion due to its greater
resistance to
weathering than the surrounding rock. Secondly, the rock was broken up into
compact-car sized (more or less) chunky pieces as it cooled and shrank. (A
basic thermal property of most solids is that they expand when heated and
shrink when cooled.) The shrinkage caused the rock to pull apart into those
chunky pieces, which have weathered into the shapes seen.

In the above picture you can see
the fracturing patterns that have allowed access by water and air into the
rock, accelerating the weathering. These fractures are called
joints.
No relative motion has occurred on either side of these fractures. Otherwise,
they would be faults.

The draw up which you are walking sees occasional flash floods due to
desert downbursts. These floodwaters carry along sand and gravel which acts
like an abrasive to wear away the bedrock. However, in many places there is
something quite bizarre about the exposed bedrock. I'm talking about
strange patterns like the ones in the image below, which was taken from above,
standing on a tall boulder.

It took a minute or two of thought, but the origin of these patterns quickly
became clear. You are looking where the erosional surface has cut through the
bedrock, exposing the jointing pattern. The discoloration is from water and the
chemicals dissolved in it seeping into the joints and reacting chemically with
the rock. This is the first step in producing the "rock spectators" that line
the trail.

A bit farther up the trail, the same patterns are seen in the rocks on the
hillsides (below).

This type of weathering, where water seeps down into rock and
weathers it into rounded shapes is called
spheroidal
weathering,
and the Grapevine Hills are a veritable celebration of this process. Not in
person nor in pictures have I ever seen spheroidal weathering so clearly
exposed and on such a scale. Below are three more pictures of spheroidal
weathering.

The first photograph is a closeup of the weathering pattern exposed on a
boulder. The second shows the process of
exfoliation,
giving birth to
new "spectators". (Exfoliation is where layers of weathered rock peel off,
sort of like peeling an onion.) Toward the top of the picture you see a boulder
pattern we dubbed "The Foot". The third is a closer picture of the
exfoliation process. (This last picture was actually taken near the end of the
trail.)

I haven't mentioned the type of igneous rock that makes up the Grapevine
Hills, although it is in several of the images.
Syenite is a fairly
rare type of igneous rock, related to
granite. However, in spite
of the close resemblance, you shouldn't take it for granite (ha! ha!), because
syenite, unlike granite, contains little or no
quartz. (Not
realizing the rock was syenite during the hike, I mistook the veins of whitish
minerals for quartz until I put my finger on a vein and had the crystals cleave
off into my hand. At that point I knew they were not quartz, since quartz does
not exhibit
cleavage. A
little dilute HCl proved the crystals were
calcite, which probably
crystallized out of groundwater in the joints.)
Syenite is found in continental settings where the
magma is high in potassium, other alkali metals, and aluminum; in syenite the
silica (silicon and
oxygen) gets used up in making feldspars, primarily
potassium feldspar (also
known as orthoclase), such that little is left over to form quartz.

At this point you have gone far enough up the trail that you might pause
and look back over the draw up which you came.

In the distance you see the Rosillos Mountains. There really isn't anything
in this picture to give
a reason for the existence of the draw at this location, but a
northwest-southeast trending fault does run through the Grapevine Hills
laccolith at this point - one of several parallel faults that do so. It is
along this fault that erosion has been accelerated, forming the draw. Later
in this virtual hike, I'll show you some evidence of the existence of this
fault. Note that the motion has been up on the southwest and down on the
northeast (as claimed by the geological map of the hills and confirmed by an
observation to be discussed later).

The goal of the hike, for many, is reaching Balanced Rock, which resides on
a ridge
at the end of the trail. This rock, like an Egyptian sovereign carried aloft by
his Nubian slaves, casts his gaze over the trail you followed in your long
pilgrimage to this august setting. (How's this for B-grade prose?)

The image above indicates a possible (but unlikely) hypothesis for the
formation of Balanced Rock. More likely it is an erosional remnant where the
rock in the "doorway" of the feature weathered away. In fact, a great example
of spheroidal exfoliation, which my brother is looking at, is found right
before you reach the rock. Also check out the boulder emerging from spheroidal
weathering just behind him.

The view above is to the south from Balanced Rock. Note the twin Spanish
daggers that serve as orientation between this picture and the one to follow.
Directly below the "d" in the word "Balanced" in the picture, you might be able
to make out a sort of miniature "Baby Balanced Rock" down the ridge on the
left. Visible in the distance to the left is an igneous
dike, a sheet-shaped
body of igneous rock that solidified in a volcanic
fissure. In the far
distance is
Nugent Mountain, a large igneous intrusion exposed by erosion. Just to the
right of Nugent Mountain you see the eastern edge of the main body of the
Chisos Mountains. In the foreground the jointing pattern responsible for
allowing such extraordinary terrain to be created is well-displayed.

"Baby Balanced Rock" is only about six or so feet long but large enough to
stand on and take the above picture of Balanced Rock from the south.

After a good morning's hike, you have returned to the beginning of the
trail, and, as you come out of the draw, you observe the trend of the fault
that led to its
creation (below). (We clambered up the boulders for this view and rested unseen
while watching late-arriving hikers make their way up the trail.)

In the distance you can see that the trend of the fault runs off into the
surrounding alluvium. The
geological map indicates a contact might be
found between two types of rock across the fault, if you can find a location
where the alluvium has been removed by erosion. On the up-faulted side you
should find igneous rock that has been exposed by uplift and erosion; on the
down-faulted side
Cretaceous rock preserved
where erosion has not yet eaten down all the way to the igneous rock.

The trace of the fault is seen above in a small arroyo into which the rare
flash-flood waters drain from the draw. On the left (south side) the uplifted
igneous rock is seen, whereas on the right you have sedimentary rock with
the igneous intrusive rock presumably at some depth below the surface.

Update: I just went back over this part of this field trip and was
embarrassed to see that I had inadvertantly written "Ahuga Formation". What?!
Maybe I was thinking of the sound old car horns make! I wrote:

The map shows the Ahuga Formation, described in the geologic
literature as sandstone and clay, should be exposed here, but the rock appeared
to be quite limy. Unfortunately, I didn't have my HCl acid bottle with me at
this point to help determine the rock type. (Limy rock fizzes when you put a
drop of dilute hydrochloric acid on it.)

The rock mapped here is actually the Aguja Formation, and another
reference I have consulted says that it can be
calcareous in part, so the
map may well be correct.